Electrocardiograph (ECG) monitors are important, and non-invasive, diagnostic medical tools. An ECG waveform is a representation of some of the electrical activity produced by a beating heart during a period of time. Two or more electrodes are placed at various location on a patient's skin and connected to an ECG monitor. Electrical signals are generated in the heart. The signals are detected on the skin by the electrodes and received by the ECG monitor. The machine amplifies and processes the signals and converts them into a representation of the heart's activity, which may be analyzed and displayed as traces on a screen, printed onto paper, or both.
It is not the intent of this discussion to provide a detailed explanation of cardiology and the analysis of ECG traces. However, a general summary is useful for background purposes. FIG. 1 is an example of a strip chart of electrical signals from a 12-lead ECG monitor connected to a patient with a normal heart. FIG. 2 identifies individual waves, intervals, and segments. While there may be some confusion or ambiguity about the labeling of different “sections” of an ECG wave, for purposes of this application an “interval” contains one or more individual wave and a “segment” connects the end of one individual wave with the beginning of the next wave. Beginning from the left side of the chart in FIG. 2, the individual waves are: the P wave, the Q wave, the R wave, the S wave, the T wave, and the U wave (which may be overlapped and hidden by the T wave and the next P wave).
Beginning again from the left side of the chart in FIG. 2, the intervals are:                a. the PR interval, from the start of the P wave to the beginning of the QRS interval;        b. the PQ interval, which if used, is the same as the PR interval when the Q wave is present;        c. the QRS interval (also known as the QRS complex), which extends from the beginning of the Q wave to the end of the S wave;        d. the ST interval, extends from the end of the S wave to the beginning of the T wave; and        e. the QT interval, is measured from the beginning of the QRS interval to the end of the T wave; and        f. the RR interval, extends from the peak of one R wave of one beat to the peak of the next R wave of the next beat.        
The segments are:                a. PR segment, extends from the end of the P wave to the beginning of the Q wave;        b. ST segment, extends from the end of the S wave to the beginning of the T wave; and        c. TU segment, extends from the end of the T wave to the beginning of the U wave.        
From the scale of the plot of FIG. 2, TABLE I may be populated with the approximate duration of approximate time, in milliseconds, associated with some of the waves, segments, and intervals.
TABLE IDuration in millisecondsName(approximate)WavesP100Q20R40S40T180U40SegmentsPR60ST140TU40IntervalsPR/PQ180QRS80ST320QT410RR750
FIG. 3 identifies the activity of the heart muscles during each of three major phases of a beat:                a. the P wave represents atrial depolarization;        b. the QRS interval represents ventricular depolarization; and        c. the T wave represents ventricular repolarization.The wave representing atrial repolarization typically occurs between the end of the P wave and the beginning of the T wave, but is typically hidden by ventricular activity.        